I’ll hold my hand up right at the start. MY mistake was MY mistake.
Yesterday I was on a run, and had to go to 2 suppliers that I’ve not visited before to make collections. From Bristol to Frome in Somerset and took the A37 then the A362 to come into Frome from the North. Approaching the outskirts of the town and there are advance warning signs telling of the weight limit in the town centre, a limit that I’m above so I start to look for the advised alternative route. get nearer and there is the alternative route to the left. hang a left and OH [zb] 12’ 9" bridge.
Get stopped about 100m from the turning and go for a walk. No where to turn round before the bridge, back to the main road (it’s about 17:15 now and dark) no way am I going to back out from there with out some help. it’s here that I notice the advisary notice that over height vehicles should go through the town in spite of the weight limit.
As I said at the start it was my mistake but having been told about the weight limit I was actively looking for the alternative route not to be advised that I could ignore this if I was over a certain height. I simply took the pictorial, mandatory circular red bordered sign as the sart of the diversionary route and not the war and peace, white on blue, directions before making a mental calculation as to what route I could take.
Eventually got backed out with the help of a rigid tipper driver who must have seen this before. If you are that driver thanks very much, it’s a good job it was dark 'cause my face was as red as the trailer livery. 
If you are a road engineer with influence in signage time to rethink this particular set of directions.
Yeah, Frome is an awkward place depending where you got to get to. 
and from of course.